DE 19634052 describes a method for controlling a push-pull output stage. The push-pull output stage has two output stage transistors which are controlled by control signals and are each assigned a sensor transistor in thermal coupling, two identical control currents being generated from the sensor currents supplied by the sensor transistors, which control currents are subtracted from the control signals of the output stage transistors.
DE 2857233 C1 describes a semiconductor power amplifier circuit with a protective circuit which is provided for protecting the output transistor from interference.
The ADSL method (ADSL: Asymmetrical Digital Subscriber Line) is a digital transmission method for twisted two-wire lines made of copper to the end subscriber in the local area for broadband applications. To date, the common signal transmission of DC voltage signals, analog voice signals and data signals has been effected in such a way that an independent signal path which is optimally designed for the respective requirements is provided for each signal component.
FIG. 1 shows such a conventional circuit concept according to the prior art. A first and second digital signal processor DSP in low-voltage technology serve for the signal processing of digital voice signals and digital data signals, respectively. The two digital signal processors DSPA, DSPB are operated with a low supply voltage VDD of +5 V, for example. The digital signal processor DSPA for the digital voice signals is connected to a voice signal driver circuit for driving the DC voltage and analog voice signals. The voice signal driver circuit contains a preamplifier VV for amplifying the low voltage amplitudes of the voice signal. The gain of the preamplifier VV is defined by the dimensioning of the resistors R1 to R4. The preamplifier VV is of fully differential construction and has two signal outputs. The two signal outputs of the preamplifier VV are each connected to the noninverting input (plus) of two driver circuits T1, T2. The signal output of the two driver circuits T1, T2 is in each case fed back to the noninverting input of the driver circuit T1, T2.
For the DC voltage voice signal transmission, the voice signal driver circuit must be able to transmit signal voltages of up to 150 V for reasons of compatibility with older telephone system concepts, for example in order to transmit ringing signals. The voice signal driver circuit is therefore produced in a high-voltage technology and operated for example from a supply voltage of +60 V at the positive supply voltage connection and −70 V at the negative supply voltage connection. The signals transmitted by the voice signal driver circuit are conventional voice signals in a frequency range of 300 Hz to 3.4 kHz with a signal amplitude of 1 V, DC voltage signals in the range of 20 20 to 100 V, ringing signals in a frequency range of 20 to 50 Hz at a voltage amplitude of 70 V and teletex signals with a frequency range of 12 or 16 kHz at a signal amplitude of 5 V.
The signal outputs of the voice signal driver circuit of fully differential construction are connected to a low-pass filter TP, which decouples data signals with a relatively high frequency.
The digital voice [sic] processor DSPB provided for the digital data signals is connected to a data signal driver circuit. The data signal driver circuit according to the prior art, as is illustrated in FIG. 1, contains a first and second line driver T3, T4. The two noninverting inputs of the two line driver circuits T3, T4 are connected to the digital signal processor DSPB. The two inverting inputs of the driver circuits T3, T4 are connected to one another via a resistor R5 and are respectively coupled to their signal outputs via resistors R6, R7. The driver circuits T3, T4 of the data signal driver circuit are connected via output resistors R8, R9 to a transformer connected downstream. The data signal driver circuit is subject to stringent linearity and signal bandwidth requirements. Therefore, the data signal driver circuit is conventionally realized using fast complementary bipolar technologies or BICMOS technologies. The complementarily constructed driver circuits T3, T4 of the data signal driver circuit have complementarily constructed driver transistors. Due to the dictates of technology, the driver circuit T3, T4 have [sic] a maximum operating voltage of ±15 V.
On account of the low operating voltages of the driver circuit T3, T4, the transmitting data signal must be stepped up to the required voltage value of 36 Vp in a frequency range of 0.13 to 1.1 MHz. For this purpose, the transformer has a primary coil L1 and two secondary coils L2a, L2b, which are connected to one another via a capacitor C. The turns ratio between the secondary coils and the primary coil is two, for example, for the purpose of doubling the data signal voltages.
The outputs of the low-pass filter TP and of the transformer are connected in parallel to the connection lines for the terminal.
The line driver circuit arrangement according to the prior art as shown in FIG. 1 has some considerable disadvantages. The digital data signals require different driver circuits in each case. Moreover, the voice signal driver circuit and the data signal driver circuit are realized in different semiconductor technologies. Therefore, integration on a semiconductor chip is possible only with difficulty and the production costs for the circuit arrangement illustrated in FIG. 1 are relatively high.
A further disadvantage of the conventional circuit arrangement for driving voice and data signals which is illustrated in FIG. 1 is that, on account of the relatively low operating voltage of the data signal driver circuit, it is necessary to provide a transformer which cannot be integrated in a semiconductor chip. Said transformer requires a relatively large amount of space and can only be produced with a relatively high outlay.
A further disadvantage is that the voice signal driver circuit has to be produced in a high-voltage technology. The high-voltage technology necessitates relatively large component dimensions which lead to high parasitic capacitances. Furthermore, the transistors embodied in high-voltage technology have relatively high layer thicknesses and are thus relatively slow.